The invention relates to an electronic trip device comprising means for performing a short delay function, designed to produce a tripping signal when an input signal, representative of the current flowing in at least one conductor to be protected, is greater than a preset threshold value over a preset time delay period, said means comprising means for comparing the input signal and the threshold value producing a signal on output when the input signal is greater than the threshold value, and time delay means connected to the output of the comparison means.
In state-of-the-art electronic trip devices, the time delays of the long and short time delay functions are achieved by means of an RC circuit. A voltage representative of the current flowing in the conductor to be protected is applied to the time delay circuit, when the current amplitude exceeds a preset short or long time delay threshold. The time delay circuit supplies a tripping signal when its output signal reaches a preset value.
U.S. Pat. No. 3,792,313 describes a circuit of this kind wherein zero resetting of the time delay circuit is delayed when the current amplitude drops back below said preset threshold. Such a time lag may be about 20 ms when the frequency of the measured current is 60 Hz, thus making the form of the current signal between two peak values irrelevant, and in particular enabling a non-filtered rectified current signal to be used.
In this type of circuit, the uncertainty as to the value of the time delay is typically about 20% to 30%, and the reliability of the circuit can be reduced by the use of high capacity time delay capacitors.